My understanding of nVidia is that it is different from ATI.
In Eyefinity, you have to set up an "eyefinity group" and designate where the 3 monitors are. I don't know if you have to do that for nVidia.
In the game, when I play in windowed mode, it's set to 1900x1000, and when I click full-screen, this is the default:
That screenshot is from 1900x1000. Of course you can quicky maximize the screen by simply enlarging the window like all windows programs. That gives you all three monitors windowed mode, but not full-screen.